Adequate delivery of blood to tissue is an essential element to maintaining health and normal tissue function. At the level of the microvasculature, this process is tightly regulated through coordinated interactions involving the autonomic nervous system, local metabolic effectors, and hormonal agents. One effect of these actions is to produce rhythmic oscillations in tissue perfusion thereby limiting the demand on the heart that would otherwise be required should continuous perfusion of tissue be required throughout the body. Modulation of this behavior is considered an important component in regulating blood pressure and in the redistribution of blood from the periphery to critical central organs during shock, among other actions. Damage to this blood redistribution mechanism is thought to be a component of a variety of disease processes, in particular, those involving autonomic dysfunction. One example is the condition known as orthostatic intolerance, wherein the normal increase in blood pressure accompanying a rise from the recumbent to standing position does not occur and results in syncope.
The details of oscillatory behaviors associated with the microvascular bed in living tissue have been studied under a variety of experimental conditions and are believed to arise from two principal mechanisms. Under neural control are the Traube-Hering-Mayer waves. These are thought to entrain large areas of tissue and serve to modulate regional changes in blood delivery to tissue. Local oscillations arise from vasomotion, which is thought to be mainly responsive to autoregulatory mechanisms. On a macroscopic level, these behaviors produce two types of phenomenology including local oscillatory behavior and propagating oscillatory behavior with the latter arising from coordinated expansion-contraction cycles. While these behaviors are widely recognized, their detailed study in intact living tissues has been mainly limited to surface examinations using the laser Doppler technique. Characterization of these behaviors in deep tissue structures could have considerable practical value but until recently has not been possible. One approach that is suitable for deep tissue studies is the method of acoustic Doppler imaging. This technique, while suitable for examining large vessels, is insensitive to the microvascular bed, which is the component of the vascular tree believed to be mainly responsible for the considered dynamics. A sensing technology is needed that is sensitive to the dynamics of the microvascular bed in deep tissues. Further, an analysis approach is needed that can define the considered phenomenology.